Chapter 25 - Electromagnetic induction Flashcards
What happens when a conductor is moved through a magnetic field? What is this called?
Cuts through the field lines, inducing an emf in the conductor
electromagnetic induction
What happens when a conductor with an induced emf is part of a complete circuit?
Induced emf forces electrons round the circuit, inducing a current
How can an induced emf be increased?
- Moving the wire faster
- Using a stronger magnet
- Making the wire into a coil
When does the induced emf become zero?
When the relative motion between the magnet and the wire ceases
How do you calculate the rate of energy transfer of energy from the source of emf to the other components of the circuit?
Induced emf x current = energy transfered per unit charge from the source x charge flow per second
= energy transfered per second from the source
What can be used to determine the direction of induced current?
Fleming’s right hand rule (Dynamo rule)
Direction of induced current is opposite to the direction of the flow of electrons in the conductor
What is magnetic flux?
Magntic flux (Ø) = BA
B = magnetic flux density and A = Area swept out
What is magnetic flux linkage?
Magnetic flux linkage through N turns of a coil = NØ = BAN
where B is magnetic flux density perpendicular to area A
= BAN (cos theta)
When the coil is turned through 180º what does the flux linkage equal?
-BAN
When the magnetic field is parallel to the coil area what does the flux linkage equal?
0 as no field lines pass through the coil area
What does faraday’s law of electromagnetic induction state?
States that the induced emf in a circuit is equal to the rate of change of flux linkage through the circuit.
What is the equation relating to Faraday’s law?
emf = -N change in flux/change in time
= - rate of change of flux linkage
What does Faraday’s law essentially tell us?
The size of the induced emf
What is Lenz’s law
The direction of the induced emf is such that it will try to oppose the change in flux that is producing it
What determines the direction of the induced current when a bar magnet is pushed into a coil connected to a meter?
Induced current passing round the circuit creates a magnetic field due to the coil. The coil field must act against the incoming north pole, otherwise it would pull the North-pole in faster - making the induced current bigger- pulling the N-pole in even faster. Due to conservation of energy (forbids creation of kinetic and electrical energy from nowhere) the induced current creates a magnetic field in the coil which opposes the incoming north pole. The induced polarity of the end of the coil (X) must therefore be a N-pole, to repel the incoming N-pole. Therefore, current must go around in an anticlockwise direction.
What direction does the induced current flow when a bar magnet is pulled out of the coil?
Induced S-pole at end X of the coil due to opposing the magnet moving away, so induced current passes around X in a clockwise direction
What is the overall explanation of Lenz’s law?
Energy is never created or destroyed. Induced current could never be in a direction to help the change that causes it; that would mean producing electrical energy from nowhere.
For a moving conductor in a magnetic field, what does the emf equal?
emf = BLdistance moved/ change iin time
emf = BLv
For a fixed coil in a changing magnetic field, what is the equation for emf?
emf = AN x change in B/change in time
For a rectangular coil moving into a uniform magnetic field, what is the equation for emf?
BNLv
Time taken for coil yo enter field completely = coil width/speed = w/v
During this time flux linkage increases from 0 to BNLw. Therefore change in flux linkage per second = BNLw/w/v = BNLv
When coil is completely in the field, the fux linkage through it does not change (A doesn’t change) - so no emf is induced
What happens when a coil in a simple ac generator rotates uniformly in a magnetic field?
Alternating emf is induced
Flux linkage giiven by BANcos(theta) and as the coil rotates, theta changes - so the flux linkage varies sinusoidally between +BAN and -BAN
What does sinusoidally mean?
Follows same pattern as a sin (or cos) graph
What is the equation for flux linkage knowing how fast theta changes depends on angular speed
NØ = BANcos(angular speed x t)
Draw the graphs of Flux linkage and emf against time for a rotating coil in a uniform magnetic field
What is the gradient of a graph of flux linkage against time?
rate of change of flux linkage = induced emf
What is the equation for alternating emf
emf = BAN(angular speed) x sin(angular speed x t)
= emf(o) sin2πft
where emf(o) is the peak emf and 2πft = angular speed
What is back emf?
emf induced in the spinning coil of an electric motor becasue the flux linkage through the coil changes.
Acts against the Pd V applied to the motor (in accordance with Lenz’s law)
V - emf = IR where I is the cirucit through the motor coil and R is the circuit resistance
Whats the equation for electrical power supplied by the source IV?
Multiply V - emf = IR by I gives IV - Iemf = I2R
electrical power supplied by the source (IV) = electrical power transferred to mechanical power (Iemf) + electrical power wasted due to circuit resistance (I2R)
What is an alternating current?
Current that repeatedly reverses it’s direction
What is the frequency of an alternating current?
What frequency does mains electricity have?
Number of cycles (moves one way, reverses and then re-reverses) its passes through each second.
50 Hz - one cycle takes 0.02 seconds in mains electricity.
What is the peak value of an alternating current?
Maximum current/Pd in either direction
Peak current in a circuit depends on the peak Pd of an alternating current source and on the components in the cicuit.
e.g. peak pd in a mains circuit is 325 V. If this pd is applied to a 100 ohm heating element, the peak current through the heating element through the heating element would be 3.25 A (325/100)
What can an oscilloscope be used for?
Display a waveform of the alternating pd from a signal generator
used to observe altenrating current
What does increasing the ouput pd from a signal generator connected to an oscilloscope do?
Makes oscilloscope trace taller. Shows peak value of the alternating pd has been made larger
What does increasing the frequency of the signal generator do when connected to an oscilloscope?
Increases the number of cycles on the screen. This is because the number of cycles per second of the alternating pd has increased.
What is the time period of an ac current?
The time taken for one complete cycle/wave
For a heating element with resistance R, what would the equation for maximum. power be?
Io2R
Where Io is peak current
Draw graph for variation of power with time for an alternating current
For a sinusoidal current, what is the equation for mean power?
1/2 Io2R
Mean power over a full cycle is half the peak power.
What is the root mean square value (of an alternating current)?
Value of direct current that would give the same heating effect/power as the alternating current in the same resistor (mean power)?
Give the equation for root mean square value of an alternating current and an alternating Pd
Irms = Io/(2)1/2
Vrms = Vo/(2)1/2
1/(2)1/2 x the peak value
Give equation for mean power supplied to a resistor in an alternating circuit
P = (Irms)2R = (Vrms)2/R = IrmsVrms
What does a transformer do?
Chnages an alternating pd to a different peak value
What does a transformer consist of?
2 coils (primary coil and secondary coil) wound around a soft iron core
How does a transformer work?
An alternating current flows into the primary coil - causing an alternatingmagnetic field to be produced in the core. Field passes through secondary coil so flux linkage of the secondary coil is constantly changing - and so ac volatge is induced across it.
Give the tranformer rule
Vs/Vp = Ns/Np
What does the time base and the voltage sensitivity/volts per division of an oscilloscope mean?
time base tells you the time represented per division/ width of square
volts per division tells you what the height of one square represents in volts
What 2 dials are on a oscilloscope?
Time base dial
Voltage sensitivity
Describe how you would use an oscilloscope to check the accuracy of the rms output voltage and the frequency of the supply. Given values are 12V rms output voltage and 50Hz frequency
power supply is connected to the input of the oscilloscope. The time base is switched off and the y gain adjusted until a complete vertical line is seen on the screen. The length of the line is measured and this is converted to peak to peak voltage using the calibration. The peak voltage is divided by root two to get the rms voltage and this is compared with the stated value. The time base is now switched on and adjusted until a minimum of one cycle is seen on the screen. The length of one cycle is measured and this is converted to time using the time base setting. Frequency is the reciprocal of this time.
